Air Quality Index is number value from the EPA ranging from 0-500 that relates to air pollution and health concern with a higher number indicating increased levels of airborne hazards.

A healthy Air Quality Index is a "good" AQI rating in the range of 0-50 with a PM2.5 concentration of 12 µg/m3 or less per the EPA. 

Air Quality Index is measured by networks of monitors that
record the concentrations of the major pollutants at
more than a thousand locations across the country each
day. These raw measurements are then converted into
AQI values using standard formulas developed by EPA.

The Air Quality Index meaning,  abbreviated AQI, is a rating the EPA gives for the level or air pollutants in a particular area such as a city or region. 

Air pollution is any mixture of solid particles and gases in the air. The EPA sets the Air Quality Index rating based on the 5 major air pollutants which are:

• ground-level ozone
• particle pollution (also known as particulate matter, including PM2.5 and PM10)
• carbon monoxide
• sulfur dioxide
• nitrogen dioxide

The EPA lists 5 major air pollutants for which the Air Quality Index is rating which are: 

• Ground-level Ozone
• Particle Pollution (also known as particulate matter, including PM2.5 and PM10)
• Carbon Monoxide
• Sulfur Dioxide
• Nitrogen Dioxide

Additionally, air pollution is caused from other sources including:

• Natural Causes
• Dust & Wildfires
• Animals & Vegetation
• Volcanic Activity
• Fossil-Fuel Emissions
• Agriculture and Animal Husbandry
• Waste

The average US Air Quality Index rating is around 40. For comparison, the worst Air Quality average in 2020 was Hotan, China with an rating of 110.2.